Methods of treating metallic powders



5 7, 1964 P. GALMICHE 3,157,532

METHODS OF TREATING METALLIC POWDERS Filed Feb. 21, 1962 UNS/NTERED ME TAL POWDER D/LUT/NG POWDER CHROM/UM HAL/0E VAPOR SOURCE I-IEA TING SUFFICIENT TO FORM CHROM/UM HAL/DE VAPOR AND CAUSE CHROM/Z/NG SEPARA TING D/LUTING POWDER FROfl/f METAL POWDER INVENTOR PHILIPPE CALM/CHE United States Patent Ofiiice 3,157,532 Patented Nov. 17, 1964 METHODS OF TREATING METALLI C POWBERS Philippe Galmiche, Paris, France, assignor to Oflice National dlltutles et de Recherches Aerospatiales called .N.E.R.A., Chatiilomsous-Bagneux, France, a society of France Filed Feb. 21, 1962, Ser. No. 174,666 Claims priority, application France Mar. 14, 1956 19 Claims. (Cl. 117l00) This application is a continuation-in-part of copending application Serial No. 645,682, filed March 13, 1957, now abandoned.

The present invention relates to methods of chromizing a powder the grains of which have as a principal component a metal of the group consisting of iron, nickel,

. cobalt, molybdenum, tungsten and alloys of at least two of said elements together, it being well understood that said principal component may be alloyed with a small amount of another metal such as aluminum or chromium. In the following description, the metal of which said powder is made will occasionally be called main metal.

My invention is more especially but not exclusively concerned with the treatment of magnetic powders and in particular of powders the cohesion of which must vary under the action of a magnetic field.

The chief object of my invention is to provide a method of this kind which is better adapted to meet the requirements of practice than those used up to the present time.

It consists essentially in heating, at a temperature at which chromium diffuses into a metal as above mentioned, in a non-oxidizing atmosphere, an intimate mixture of a powder of said metal with a powder consisting at least partly of chromium, and evolving vapors of at least one chromium halide in said heated mixture, under conditions where said powder mixture cannot be sintered.

Other features of my invention will become apparent in the course of the following detailed description thereof.

It is of great interest to protect metallic powders as above specified against oxidation, in particular at high temperatures, and for this purpose every grain of the powder must be uniformly and evenly protected because, otherwise, a superficial defect will cause oxidation of the mass of the grains and will lower the magnetic permeability of the powder.

It has already been proposed to form on the surface of the grains of such a metallic powder a superficial diffusion alloy by means of vapors of a halide of an addition metal such as chromium, but, up to the present time, the methods that have been proposed in order to effect such a treatment did not permit of obtaining a sufficient diffusion of addition metal equal for all the grains and which can be reproduced exactly.

In the drawings, the sole figure is a flow sheet of the process in accordance with my invention.

Furthermore, in some prior methods, the powder grains had a tendency to agglomerate by sintering, which is of course disadvantageous because, in order to have a magnetic powder the cohesion of which is controlled by the action of a magnetic field, the individual grains must remain independent of one another.

According to my invention, I form a mixture of the metallic powder to be treated with a powder consisting at least partly of chromium, and I heat this mixture in a non-oxidizing atmosphere, at a temperature at which chromium diflfuses into the metal of the powder, while evolving vapors of at least one chromium halide in said mixture, under conditions where said powder mixture cannot be sintered.

In order to prevent sintering, I may perform the treatment by limiting the heating to a temperature lower than the sintering temperature of the mixture.

However, better results will be obtained at higher temperatures, and in this case I incorporate in the mixture a diluting powder, preferably of a grain size lower than that of the chromium powder.

By the use of such a diluting powder, which separates from one another the grains of the mixture, I prevent not only sintering but also mere sticking of the grains to one another and I am able to introduce into the mixture, without any drawback, the desired halides and to obtain a good circulation of the halide vapors through said mixture.

When such a diluting powder is used it will be necessary, at the end of the treatment, to separate from the treated powder, the diluting powder, and also possibly the amount of chromium powder that has not diffused into the main metal.

The atmosphere in the container where the treatment is carried out must be a neutral or reducing atmosphere; for instance I may use for this purpose an atmosphere of ammonia which has been previously cracked or which is cracked in the container. I may also use a mixture of hydrogen and nitrogen obtained by a partial combustion of ammonia, followed by dehydration.

In this description and in the appended claims, the term powder, as applied to the metal powder to be treated is restricted to the case of powders of a particle, or grain, size up to 300 microns.

The chromium powder which is mixed with the main metal powder is advantageously as fine as possible, and its grain size should range from 5 to microns. Advantageously, it is finer than the powder of metal to be treated, forv instance from five times to twenty times finer.

The chromium halide vapors by means of which chromium is conveyed to the grains of the main metal, so as to diffuse thereinto, may be obtained indirectly from a source located out of contact with the powder mixture and heated together therewith in the same container.

The chromium halide vapors are evolved from the inside of the powder mixture. This way of proceeding ensures a very good homogeneity of the layer of difiusion alloy, even when the amounts of metallic powder in treatment are very large.

When the chromium halide vapors are obtained from a source located at a distance from the powder mixture, this source must be capable of giving oil a volatile metallic halide capable of exerting a progressive action upon the chromium contained in the powder mixture.

It should be well understood that, in the present specification, the term metallic halide includes not only halides of metals proper but also the halogen acids such as hydrochloric acid, hydroiodic acid, hydrobromic acid and bydrofluoric acid, which may be considered as halides of hydrogen, and also ammonium halides and the halides of amines containing at most three carbon atoms for one halogen acid.

It should be noted that when the chromium halide is formed in the powder mixture, it is necessary to determine a given proportion between the areas of the chromium powder and the areas of the main metal powder which are subjected to the action of the halide intended to give birth to the chromium halide. Of course, this proportion depends both upon the relative weights of main metal powder and chromium powder mixed together and upon the respective grain sizes of said powders. This necessity is due to the fact that the halide vapors which act upon the chromium grains to form the desired chromium halide also act upon the grains of the main metal (for instance iron), so as to form an iron halide. This last mentioned formation slows down the'difiusion of chromium due to the fact that a balanced reaction takes place and also that iron is diffused on the chromium grains. In particular, there may be produced a reduction of the iron halide which deposits, at theend of the heating up period, on the iron grains in the form of scales which are little adhesive to said grains. I v

When iron powder is .treated with. a chromium powder finely divided and the amount of which is relatively high (from to 3O% for instance), the production of iron halide is not' 'very detrimental. The total area of the chromium grains is very large, and as chromium in attacked before iron by the halide, it absorbs most of the acidity.

On the contrary, when powders which are already protected by means of-chromium against oxidation (for instance ferro-chrornium powders or term-aluminum powders) are chromized, in which case the amount of chromium is much lower, the above indicated drawback is rn'ore to be feared;

As a rule, it is necessary to avoid too high a disengage- 'ment of halide of the main metal." However, in some cases, it may be of interest to obtain a slight attack of the grains of the main metal in order to round olf said grains and to achieve a kind of chemical polishing thereof. I

When the chromium halide vapors are supplied from a source incorporated in the powder mixture, this source maybe said chromium halide itself in the solid form, which upon heating gives off the desired vapors. Alternately, the chromium halide vapors may be formed in the powder mixture by a reaction of another halide upon the grains of the chromium powder incorporated in the powder mixture. Said other halide need not be so volatile as when the source is located on the outside of the powder mixture.

In this case, the same precautions as above mentioned must be taken in order toavoid the formation of too high an amount of the main metal halide. This formation is the smaller as the amount of chromium in the mixture is greater and as said chromimum is in the form of a finer powder. The effects of such a formation of main metal halide are not so detrimental if thehalide is a :fiuoride because, in this case, the initial formation of 7 iron, nickel or cobaltfluoride is limited and chromium is conveyed to the grains of iron, nickel or cobalt with a disengagement of hydrofluoric acid in a hydrogen atmosphere which does not slowdown the reaction.

Therefore, from this point of view, it is advantageous to make use of'a fluoride to produce the desired chromium halide vapors.

The. fact that the powder mixture contains chromium in astate of very time division provides, around the grains of the main metal, a multiplicity of points. of formation of chromium halide, said points being evenly distributedin close proximity to the grains to be chromized. In this case, the fact that the chromium halide is not very volatile is not a drawback and the attack of the main metal by the halide is very much reduced.

The chromium halide to be incorporated in the powder mixture is advantageously obtained as follows.

In a preliminary operation I heat, for instance during minutes at 500 C., a fine chromium powder mixed with ammonium halide and with the inert diluting powder to be used. This operation supplies a mixture of chromium halide, chromium and diluting powder. This mixt-ure may be stored up so as to be used in p edetermined amount, possibly after it has been analyzed.

It should be noted that, in the case of fluorides, which are not hygroscopic, this storing up involves no difiiculty. In the case of the other halides, the fact that an inert diluting'powder is mixed with the halide prevents to some degree an agglomeration of the halide grains together.

In 'order to prevent any sintering of the powder mixture, I may operate at temperatures substantially lower than the sintering temperatures, for instance equal to at most 800 C., if the diffusion layers to be obtained are relatively thin' (averaging one micron), or operate at temperatures where sintering of the metal powders might take place, but, in this case, a diluting powder must be incorporated into the powder mixture.

The inert diluting powder incorporated in the powder mixture may contain a metallic oxide, provided that the metal of this oxide does not'diifuse in the main metal of the powder. Preferably, the inert diluting powder is of a grain size as small as possible, and preferably smaller chromium halide (for instance A1 0 or SiO or ZrO The amount of diluting powder that is incorporated in the mixture advantageously ranges from 10 to 50% of the total mass, and preferably averages 25% thereof by weight.

Whatever be the solution that is adopted,the treatment temperatures will range from 450 to 1300 C. and, preferably, from 800 to 1100 C., the higher temperatures being generally used for the treatment of materials which already contain some amount of chromium (ferrochromium).

The atmosphere existing in the treatment container is preferably a reducing atmosphere. For instance, it may contain hydrogen formed in situ (ammonium halide) or coming from the atmosphere existing around the container, in which case the container is not closed in a wholly fiuidtight manner, so as to permit communication with the space where said hydrogen is present.

Due to the fact that the chromium halide vapors are formed and/or regenerated in close proximity to. the grains of the metal, these last mentioned'grains are immersed in vapors of chromium halide transmitted from a multiplicity of points very close to said main metal powder grains, which permits of obtaining a perfectly regular chromizing at a relatively low temperature and without any risk of agglomeration. I

In order to be sure that the powders are treated in a manner which always gives the same grains treated powder, I advantageously proceed as follows, according to a very important feature of my invention.

1 mix with the metal powder to be treated an amount of chromium that is carefully determined so that the whole of this chromium has diffused in the metal powder to be treated at the end of the operation.

Of course, this determination of the amount of chromium just necessary in order to obtain the desired result is made in accordance with the temperatures and duration of the diffusion treatment.

Advantageously, the chromium powder has a very fine grain size. Preliminary tests will be made in order to obtain the desired indications. I

The heating treatment may be maintained after the whole of the chrominum present in the powder has dif-. fused into the main metal grains, in order to obtain in every grain of said metal a deeper penetration of chromium toward the center of the grain, whereby the thickness of the diffusion alloy layer is increased and the percentage of chromium at the surface is reduced. Such an operation may be preformed, in'particular, in order to eliminate the sigma phase in steel grains.

When the amount of chromium is limited in the powder magnetic permeability and might also have a detrimental abrasive action.

It must also be pointed out that when an excess of chromium has been incorporated in the powder mixture, it is dangerous to reutilize the excess that may have been recuperated from a treated metal powder in order to carry out another treatment, because said chromium powder thus recuperated may be soiled and, in particular, may contain some amount of the main metal.

However it may happen that it is necessary, for instance if no sufficiently fine chromium powder is available, to introduce an excess of chromium powder in the powder mixture. In this case, after the heating treatment, there is obtained a mixture constituted by the chromized main metal powder, an excess of chromium powder, the diluting powder and also iron chromium halides.

It is therefore necessary to eliminate the excess of chromium powder and the diluting powder. Even when there is not an excess of chromium, it is always necessary to eliminate the diluting powder. Elimination of the halides involves no difiiculty because they are easily soluble in water.

Separation of the excess of chromium powder may be obtained, for instance in one of the following manners:

(a) By sifting when the chromium powder is of a grain size sufiiciently different from that of the chromized metal powder, or by having the chromium powder (which is much finer) carried along by a stream of water;

(b) By taking advantage of the difference of density of the chromium powder and of the main metal powder when such a difference exists, in which case I may for instance proceed by cent-rifugation of the mixture of said two powders;

(c) Advantageously, by a magnetic method, since the treated metal powder is generally magnetic whereas chromium powder is not.

The diluting powder may be eliminated in one of the following manners:

Through physical means, when said diluting powder is of a density sufficiently different from that of the main metal powder, for instance iron powder (centrifugation or action of a stream of water for instance),

Or also, and at least partly at the end of the operation, chemically when the diluting powder is soluble in a reagent which does not attack chromium or alloys of chromium with the main metal, which is the case when the diluting powder is constituted by magnesia which is sub sequently eliminated by attack by means of nitric acid in solution in water.

The treatment according to my invention may be carried out in conditions leading to the formation of diffusion layers of chromium carbides, when the main metal is provided with a superficial carbide layer. In this case, the chromium halide is obtained from an ammonium halide directly incorponated in the powder mixture.

On the other hand, a chromized iron powder obtained according to my invention may be subjected to a nitriding treatment in a rotary furnace, this operation being carried out at temperatures ranging from 300 to 700 C. during a time which may range from some minutes to some hours.

When the halide that is used is not a fluoride, it is of interest to oxidize the powders at the beginning of the treatment in order to reduce the risks of attack of said powders by halide vapors just at the beginning of the operation. This preliminary oxidizing may be carried out on the spot before the chromizing period proper, either by incorporating compounds such as ammonium carbonate into the powder mixture, or by making use as diluting powder of a compound such as magnesium carbonate.

1 will now indicate an interesting feature of my invention in order to obtain a diffusion alloy layer which is both thin and evenly distributed on every grain of the powder.

This feature consists in giving the chromium powder a grain size smaller than that of the main metal, the

grain size of the chromium powder ranging preferably from 5 to 30 microns.

In some cases, after the grains of the powder to be treated have been chromized, it is advantageous to effect a protective superficial oxidizing of the chromium of said grains before the powder is placed in the apparatus where it is to be used.

This selective oxidizing is carried out in a stream of hydrogen containing traces of oxygen or water vapor, at temperatures ranging from 500 to 750 C., the operation being elfected in times ranging from some minutes to some hours.

This operation makes it possible to stabilize initially the magnetic permeability.

I will now give some examples of the method according to my invention.

Example .1.--A ferro-chromium powder containing 13% of chromium, of regular grain size (mean diameter of the grains of about microns) is mixed with 5% by weightof metallurgical chromium crushed so that its grain size averages 20 microns and with 15% of a powder of magnesia having a grain size averaging about 20 microns. To this mixture there is intimately mixed 5% of a chormium chloride cementation mixture prepared separately by heating, for 30 minutes at 500 C., a mixture of magnesia (23%), of chromium powder of a grain size lower than 40 microns (72%) and of ammonium chloride, or bromide, or iodide (5%).

The whole is intimately mixed together, then heated for an hour at 1025" C., preferably in an atmosphere of cracked ammonia and in partly fluidtight boxes, such a treatment resulting in a dilfusion of chromium into the grains and in a deeper penetration of the chromium layer toward the center of the grains without external addition of chromium, due to the fact that the amount of chromium has been deliberately chosen relatively small.

After cooling, magnesia and the halides that may remain are dissolved in a solution of nitric acid at 20% at a temperature of about 50 C. and the chromized iron powder that remains is then washed in water.

After drying, I obtain a chromized iron powder the grains of which are uniformly coated with chromium with a very regular gradient of the percentage of chromium from the surface toward the center, the mean percentage of chromium in the grains having been increased from 13% (before chromizing) to 15.2% (after chromizing) and the magnetic permeability of the powder remaining substantially the same as before the treatment.

It has been found that the protection against oxidation (and therefor the constancy of the magnetic characteristics of the chromized powder) remained efi'icient for very long times at temperatures which may be higher than 800 C.

Example 2.-I mix an iron powder the grains of which have a mean diameter averaging 80 microns to 20% by weight of crushed chromium such as above defined and to 20% by weight of magnesia. To this mixture I add and mix intimately 2% of chromium chloride. The whole is heated for one hour and a half at a temperature of 950% C. and the chromized powder is separated as in the preceding example. The chromized powder grains are coated with a very regular diffusion alloy layer of 0.0175 mm., the average percentage of chromium in the grains being 14.75. As in the preceding example, protection against dry corrosion is obtained up to temperatures averaging 800 C., and the magnetic permeability of the powder varies but very slowly when it is used. Besides, said powder has no substantial remanence.

Example 3.-This example diifers from the preceding one by the fact that the halogen cementation mixture contains 13% of chromium and 4% of aluminum in powder.

The powder obtained after treatment contains an average of 11% of chromium and 3% of aluminum. It resists dry oxidation for long periods of time at temperatures which may be as high as and even higher than 850 C.

Example 4;I mix iron powder ofa grain size equal to 20 microns with 20% of chromium powder of similar grain size and I heat the Whole for 20 minutes at 820 C. in the presence of 2% of ammonium fluoride. Separation of the chromized iron powder is performed magnetically. I thus obtain an iron powder which is chromized regularly with a thickness of 2.5 microns.

Example -5.I intimately mix an iron powder the grains of which have one dimension scmaller than 500 microns with 25% of chromium powder of a grain size of 100 microns and 50% of very fine magnesia. The treatment is carried out at 900 C. for one hour in'the presence of 2% of ammonium chloride. The whole is heated in partly fiuidtight boxes placed in a hydrogen atmosphere. Magnesia is dissolved by nitric acid and the residual chromium powder is separated magnetically from the chromized iron powder. The grains of chroinized iron powder that are thus obtained are of round shape and they do not oxidize at temperatures up to 550 C. The diifusion layers are perfectly regular and the grains that are obtained are very magnetic but less remanent than iron grains of analogous dimensions. This chromiz ed powder might initially be oxidized superficially as above stated in a stream of industrial hydrogen containing traces of oxygen at 750 C. for one hour.

Example 6.-I proceed as in Example 5, but instead of iron powder I make use of ferro-niclcel.

Example 7.I proceed as in Example 5, but the chromized iron powder is finally nitrided by means of ammonia in a rotary oven for 15 minutes at 475 C.

according to which, instead of directly incorporating a a chromium powder in the powder structure, this chromium powder is formed by reduction of chromium oxide (Cr O or possibly of any oxygen containing compound of chromium such as a carbonate or'an oxalate, this reduction being effected by means of a metal compound containing at'least one alkaline earth metal, this metal being preferably magnesium. In some cases, I may also make use of an alkali metal.

The reduction reaction is as follows:

This reaction, which is highly exothermic, starts at a temperature of about 600 C., and if no special precautions were taken, it would lead to very high temperatures, finally higher than the boiling point of the chromium halide by means of which the main metal grains are 'chromi zed.

v According to my invention, I suitably choose'the composition and the mass of the mixture in which this reactiontakes place in such manner that the heat given 01f by exothermic reduction reaction merely brings the temperature of said mixture to a value substantially lower than the boiling point of the above mentioned chromium halide.

The mixture is subjected to a complementary heating in order to maintain its temperature at a substantially constant value (this temperature being that chosen for g the treatment) lower than the boiling point of the chromium halide and such that the vapor tension of said chromium halide is sufficient to ensure a substantial'diffusion of chromium into the grains of the powder to be treated.

When'said powder to be treated is a molybdenum or tungsten powder, the temperatures necessary for diffusion may be as high as 1300 C., which is above the boiling point of the chromium halide. It is then advantageous to keep at least a portion of the treatment container at a temperature lower than said boiling point so as to form a reserve which acts by its vapor tension.

Use of magnesium as reducing agent according to my invention has many advantages the most important of which are as follows:

The reduction reaction takes place in the mixture at the beginning of the treatment and within a temperature range which is below the temperature at which the chromium halide vaporizes;

v The reduction reaction is extremely regular throughout the whole mixture, because magnesium is volatile at atmospheric pressure at temperatures above 600 C. and it boils at a temperature close to 900 C., which permits a perfect distribution of magnesium vapors and their utilizationis a practically quantitative fashion;

Magnesium halides are very little volatile (their boiling points being always higher than 1000 C.), which avoids a loss or" the halogen compound; I

Magnesium can give rise to no undesirable diifusion reaction with the other metallic components of the mixture, and in particular it does not diffuse into chromium or iron; a

The reduction reaction is quantitative, that is to say takes place in a complete manner due to the fact that the heat of formation of magnesium oxide is much higher than that of chromium oxide;

The reduction reaction gives rise to the formation of a by-product (magnesium oxide) which acts as a very useful diluting body during the treatment, this by-product being easy to eliminate by means of nitric acid.

The chromium oxide that is used for the reduction reaction is the commercial oxide which exists in the form of an extremely fine powder, whereas magnesium may be in any form whatever (shavings, filings, etc.) since it reacts in the vapor state.

If it is not necessary, at the end of the treatment, to separate by dissolution or chemical action the powder that has been treated from the residual elements (case of powders of relatively large dimensions which may be separated by physical separation or magnetically), I may make use of an excess of chromium oxide with respect to the amount of magnesium that is used. This excess of chromium oxide plays the part of a complementary amount of diluting powder. In the contrary case, I make use of amounts of these two bodies corresponding preferably to the stoichiometric ratio of the reduction reaction, said amounts being such that the desired amount of chromium powder is finally obtained.

In other cases I may provide a slight amount of magnesium so as to be sure that the whole of the chromium contained in the oxide is utilized.

It should be noted that, in the mixture which is subected to heating, there are elements which, although they do not participate in the reduction reaction, have nevertheless a useful effect during this reaction because they absorb the heatwhich is given off during this exothermic and said amounts are distributed in a very homogeneous manner inside the total mixture.

When the said mixture contains a volatile halide (such as an ammonium halide) intended to form the corresponding halide of chromium by attacking the chromium powder, said volatile halide reacts (at the same time as the fine chromium powder is being formed), both on said chromium powder and upon the powder of the main metal, so as to form a chromium halide and a halide of the main metal, this last mentioned halide being less volatile than the initial halide (ammonium halide), which will give rise indirectly to the formation of chromium halide by balanced exchange reaction.

If it is desired to obtain, on the grains of the powder to be treated, mixed diffusion layers, that is to say layers of chromium and silicon, or chromium and aluminium, or chromium and zirconium, I proceed as follows.

The chromium powder is formed in a separate preliminary operation by heating a mixture of chromium oxide (X 0 magnesium and magnesia (this last mentioned body being introduced so as to prevent the reaction from becoming too violent). This reduction gives a mixture of chromium powder and magnesia forming a diluting powder. The auxiliary addition metal or metals is then added, in the form of sflicon powder, aluminium powder or Zirconium powder to this mixture of chromium and magnesia.

(3f course, this way of proceeding, according to which chromium oxide is reduced during a preliminary step, may also be applied in the case of simple chromizing (i.e., without diffusion of an auxiliary metal such as above mentioned), the only drawback being that the method includes two successive steps instead of only one.

On the other hand this method in two steps has the advantage that at least part of the diluting powder necessary for the treatment is produced (in the form of mag nesium oxide resulting from the reduction reaction) during this preliminary step.

According to still another feature of my invention, when it is desired to reduce the richness of the diffusion ailoy layers in chromium (for instance in order to avoid a substantial lowering of the magnetic permeability of the grains of the powder to be treated), I use, as halide incorporated in the mixture in which the reduction reaction takes place, a halide of said metal to be treated, and in particular an iron halide. This feature may be applied both in the case where the chromium powder is obtained from a reduction operation and in that where chromium is introduced directly in the form 'of a chromium powder, possibly mixed with ferro-chromium powder.

In what precedes, it has been supposed, by way of example, that magnesium constitutes the reducing metal, but I mi ht use, possibly under reduced pressure (either separately or in combination with magnesium) other alkaline-earth metals (calcium, strontium, barium, etc.) or alkali metals (such as sodium, potassium, rubidium, etc.) which more or less possess advantages of the kind above mentioned.

However it should be pointed out that the use of alkali metal requires a substantial heating and a great excess of metal, in particular due to the low boiling point of such metal (about 300 C. for sodium) and to the lower formation heat of their oxides. The other alkaline-earth metals may be used but they give satisfactory results chiefly when they are used in combination with magnesium, or under reduced pressure, or for the treatment of highly refractory metals or alloys which require very high temperatures for diffusion.

I will now give examples of my invention.

Example 9.I form in a partly fiuidtight box a homogeneous mixture consisting of:

1000 g. of iron powder of a grain size ranging from to 800 microns and preferably from 100 to 150 microns,

to g. of magnesium in the form of filings or shavings,

325 g. of chromium oxide,

180 g. of magnesia (to which is added during the operation the magnesia resulting from the reduction of chr0- rnium oxide),

25 g. of ammonium chloride or fluoride.

This box is heated for 2 hours at 1000 C. (this heating being conducted in such manner that the mass remains for a sufiicient time within the temperature range corresponding to the reduction reaction), then I proceed to a separation by washing and to the elimination of traces of magnesia by means of nitric acid (the chromium that is formed is wholly utilized and there is no question of separating it).

I thus obtain 1100 g. of iron powder evenly chromized over a thickness of 25 microns with an average percentage of chromium of 15.5% in the case where ammonium chloride has been used, and with a. difiusion layer a thickness of 30 microns with an average percentage of chromium of 17.1 when ammonium fluoride is used.

Example 10.When it it desired to obtain very fine chromizing layers, I use iron chloride as halide added to the powder. If the same amounts as indicated in Example 9 are used (ammonium chloride or fluoride being replaced by iron chloride), I obtain chromized layers of 20 microns with an average percentage of chromium of 13.6.

Example 11.--This example differs from Example 9 merely by the substitution, for ammonium chloride or fluoride, of a mixture containing equal amounts of ammonium chloride and ammonium bromide, the results being identical to those of Example 9.

Example J2.I form in a partly fluidtight box a homogeneous mixture of 1000 g. of molybdenum of a grain size averaging 50 microns,

From 160 to 180 g. of magnesium in the form of filings or shavings,

325 g. of chromium oxide,

200 g. of magnesium oxide (to which is added the magnesia formed during the reduction of chromium oxide),

25 g. of ammonium chloride.

The whole is heated for 2 hours at 1250 C., then the chromized layer is separated as in the case of Example 9. I thus obtain 1050 g. of molybdenum powder very evenly chromized with a thickness of the chromized layer 1000 g. of iron powder of a grain size of 300 microns, 215 g. of magnesium,

375 g. of chromium oxide 150 g. of magnesia,

25 g. of ammonium fluoride.

This mixture is heated for 2 hours at 1125" C., and after separation I obtain 1200 g. of chromized iron powder the grains of which have an average percentage of chromium of 19.5.

Of course, such sintered articles might be obtained from chromized powders made according to other embodiments of my invention.

' 30 g. of ammonium bromide.

1 1 Example 14.I from in a partly fluidtight box a homogeneous mixture consisting of: 1000 got tungsten of a grain size close to 50 microns, 356 g. of magnesium in the divided state, 325 g. of chromium oxide, 250 g. of calcium oxide (CaO) 15 g. of ammonium iodide (chromium iodide has a very high boiling point close to 1400 C.). I The. whole is heated for 2 hours at 1350 C, and I obtain a chromized powder with a thickness of the chromized layer equal to '10 microns and an average percentage of chromium of 20.

Example 15.-I heat in a partly fluidtight box a ho- 300 g. of a mixture of calcium and magnesium containing 75% of calcium,

325 g. of chromium oxide,

290 g. of magnesium oxide,

I heat for 3 hours at 1150 C. and I obtain a chromized powder in which the layer is microns thick.

My experiments have shown that, in order to obtain relatively fihe layers of chromium alloys, it is advisable to use a volatile halide consisting of iron halides, whereas in order to obtain thick and rich layers of chromium it is advisable to make use of a fluoride.

The formation of chromium by reduction of chromium oxide by means of magnesium permits of reducing the cost of the chromium powder to about one-half of that corresponding to the case where chromium is obtained by crushing metallurgical chromium.

What I claim is:

1. A method of treating an unsintered powder the grains of which have as a principal component a metal of the group consisting of iron, nickel, cobalt, molybdenum, tungsten, and alloys 0:". at. least two of said elements together, which method comprises heating, at a temperature at which chromium dififuses into said metal but below the melting points of chromium and said metal, in a nonoxidizing atmosphere, an intimate mixture of said powder with at least one substance capable, at least when heated below said temperature, of giving chromium powder, with a diluting powder of a substance chemically inert with respect to chromium and to said metal and capable of resisting theeifects of heating at said temperature while preventing sintering of the grains of the two first mentioned powders, and with a powder of at least one halide capable of forming, at said temperature, within said mixture, vapors of at least one chromium halide from which chromium diffuses into the whole surface of said metal grains, thus chromizing said unsintered'powder, and separating said diluting powder from the chromized powder after the heating step.

2. A method according to claim 1 wherein said diluting powder is selected from the group consisting of magnesia, lime and graphite.

3. A method according to claim 1 which comprises still heating the powder treated after the whole of said chromium has diffused thereinto.

4. A method according to claim 1 in which the amount of diluting powder ranges from 25 to 50% of the total mass in treatment.

v5. A method accordingto claim 1 which further comprises separating chromium in excess from the treated mass after the heating treatment.

6. A method according to claim 1 which further cornprises nitriding the treated mass after the heating treatment. 1 1

7. A method according to claim 1 in which said powder to be treated is subjected to a preliminary oxidizing treatment.

8. A method according to claim 1 in which said halide is a halide of the metal of which is made the unsintered powder.

together, which method comprises heating, at a. temperature at which chromium diffuses into said metal but below the melting points of chromium and said metal, in a non-oxidizing atmosphere, an intimate mixture of said powder with a chromium powder, with a diluting powder of a substance chemically inert with respect to chromium and to said metal and capable of resisting the effects of heating at said temperature while preventing sintering of the grains of the two first mentioned powders, and with.

a powder of at least one halide capable of forming at said temperature, within said mixture, vapors of at least one chromium halide from which chromium diffuses into the Whole surface of said metal grains, thus chrornizing said unsintered powder, and separating said diluting powder from the chromized powder after the heating step.

11. A method according to claim 10 in which the grain size of said chromium powder is smaller than that of the metal powder treated.

12. A method according to claim 10 in which the grain size of said chromium powder ranges from 5 -to microns.

13. A method according to claim 10 in which the grain size of said chromium powder ranges from 5 to 30 microns. p

14. A method according to claim 10 in which the amount of said chromium powder and the grain size thereof are chosen so that at the end of the treatment the whole of said. chromium has diffused into the grains of the powder treated.

15. A method according to claim 10 in which the mixture contains, mixed therewith, some amount, of a substance of the group consisting of aluminium, silicon and zirconium. 1

16. A method of treating an unsintered powder the grains of which have as a principal component a metal of the gronpconsisting of iron, nickeL-coba-lt, molybde nuin. tungsten, and alloys of at least two of said elements together, which method comprises heating, at a temperature at which chromium diffuses into said metal but below the melting points of chromium and said metal, in a nonoxidizing atmosphere, an intimate mixture of said powder with a powder of an oxygen containing chromium compound, with fragments of at least one metal of the group consisting of alkali metals and alkaline earth metals, with a diluting powder of a substance chemically inert with respect to chromium and to said metal and capable of resisting the effects of heating at said temperature While preventing sintering of the grains of the two first mentioned powders, and with a powder of at least one halide capable of forming, at said temperature, within said mixture, vapors of at least one chromium halide from which chromium diffuses into the Whole surface of said metal grains, whereby, during a first step of said heating, said oxygen containing chromium compound is reduced, in situ, into chromium powder distributed in said intimate mixture of powders and, during a second step, said chromium powder reacts to form a chromium halide, thus chromizing said unsintered powder, and separating said diluting powder from the chrornized powder after the heating step. 7

17. A method of treating an unsintered powder the grains of which have as a principal component a metal of the group consisting of iron, nickel, cobalt, molybdenum, tungsten, and alloys of at least two of said elements together, which method comprises heating, at a temperature oxidizing atmosphere, an intimate mixture of said powder with a powder of a chromium oxide, with fragments of magnesium, with a diluting powder of a substance chemically inert with respect to chromium and to said metal and capable of resisting the effects of heating at said temperature While preventing sintering of the grains of the two first mentioned powders, and with a powder of at least one halide capable of forming, at said temperature, within said mixture, vapors of at least one chromium halide from which chromium diffuses into the whole surface of said metal grains, whereby, during a first step of said heating, said chromium oxide is reduced, in situ, into chromium powder distributed in said intimate mixture of powders and, during a second step, said chromium powder reacts to form a chromium halide, thus chromizing said unsintered powder, and separating said diluting powder from the chromized powder after the heating step.

18. A method of treating an unsintered powder the grains of which have as a principal component a metal of the group consisting of iron, nickel, cobalt, molybdenum, tungsten, and alloys of at least two of said elements together, which method comprises heating, at a temperature at which chromium diffuses into said metal but below the melting points of chromium and said metal, in a nonoxidizing atmosphere, an intimate mixture of said powder with a powder of a chromium oxide, with fragments of magnesium, with a diluting powder of magnesia, and with a powder of at least one halide capable of forming, at said temperature, within said mixture, vapors of at least one chromium halide from which chromium diffuses into the whole surface of said metal grains, whereby, during a first step of said heating, said chromium oxide is reduced in situ, into chromium powder distributed in said intimate id mixture of powders and, during a second step, said chromium powder reacts to form a chromium halide, thus chromizing said unsintered powder, and separating said diluting powder from the chromized powder after the heating step.

19. A method of treating an unsintered powder the grains of which have as a principal component a metal of the group consisting of iron, nickel, cobalt, molybdenum, tungsten, and alloys of at least two of said elements together, which method comprises heating, at a temperature at which chromium diffuses into said metal but below the melting points of chromium and said metal, in a non-oxidizing atmosphere, an intimate mixture of said powder with a powder of a chromium oxide, with fragments of magnesium, with a diluting powder of calcium oxide, and with a powder of at least one halide capable of forming, at said temperature, within said mixture, vapors of at least one chromium halide from which chromium diffuses into the whole surface of said metal grains, whereby, during a first step of said heating, said chromium oxide is reduced, in situ, into chromium powder distributed in said intimate mixture of powders and, during a second step, said chromium powder reacts to form a chromium halide, thus chromizing said unsintered powder, and separating said diluting powder from the chromized powder after the heating step.

References Cited in the file of this patent UNITED STATES PATENTS 2,657,127 Sinderband Oct. 27, 1953 FOREIGN PATENTS 598,181 Great Britain Feb. 12, 1948 693,292 Great Britain June 23, 1953 

1. A METHOD OF TREATING AN UNSINTERED POWDER THE GRAINS OF WHICH HAVE AS A PRINCIPAL COMPONENT A METAL OF THE GROUP CONSISTING OF IRON, NICKEL, COBALT, MOLYBDENUM, TUNGSTEN, AND ALLOYS OF AT LEAST TOW OF SAID ELEMENTS TOGETHER, WHICH METHOD COMPRISES HEATING, AT A TEMPERATURE AT WHICH CHROMIUM DIFFUSES INTO SAID METAL BUT BELOW THE MELTING POINTS OF CHROMIUM AND SAID METAL, IN A NONOXIDIZING ATMOSPHERE, AN INTIMATE MIXTURE OF SAID POWDER WITH AT LEAST ONE SUBSTANCE CAPABLE, AT LEST WHEN HEATED BELOW SAID TEMPERATURE, OF GIVING CHROMIUM POWDER, WITH A DILUING POWDER OF A SUBSTANCE CHEMICALY INERT WITH RESPECT TO CHROMIUM AND TO SAID METAL AND CAPABLE OF RESISTING THE EFFECTS OF HEATING AT SAID TEMPERATURE WHILE PREVENTING SINTERING OF THE GRAINS OF THE TWO FIRST MENTIONED POWDERS, AND WITH A POWDER OF AT LEAST ONE HALIDE CAPABLE OF FORMING, AT SAID TEMPERATURE, WITH SAID MIXTURE, VAPORS OF AT LEAST ONE CHROMIUM HALIDE FROM WHICH CHROMIUM DIFFUSES INTO THE WHOLE SURFACE OF SAID METAL GRAINS, THUS CHROMIZING SAID UNSINTERED POWDER, AND SEPARATING SAID DILUTING POWDER FROM THE CHROMIZED POWDER AFTER THE HEATING STEP. 